This Ball Roller Coaster will keep two balls busy
rolling down the track over and over again. The ultrasonic sensor
tells the NXT when to use the lift motor to lift a ball back up to the
top of track, so the roller coaster operates fully automatically on its
own.

If you have a good supply of extra parts, then this
project has a lot of potential for additions and extensions. How
big and interesting of a track can you make?